fraud, maybe not. bad idea, maybe. the idea was that a lot of effort and money was spent certifying a plane, and it makes sense to allow delta changes to the design without revisiting the entire process. so a plane is in service and customers want it to fly a longer distance. the oem makes them a configuration that has more fuel tanks, and less seats. the delta change has to be evaluated against the original certification, because it is possible that meeting current rules is impossible without major rework (think how wiring is done, the kind of hydraulic fittings allowed, things that are everywhere. it allows the economic life of the design and the investment of end users (airlines) to be protected with some semblance of regulation.